A compound semiconductor is a compound that is composed of at least two types of elements rather than one type of element such as silicon or germanium and operates as a semiconductor. Various types of compound semiconductors have been developed and are currently being used in various fields of industry. Typically, a compound semiconductor may be used in thermoelectric conversion elements using the Peltier Effect, light emitting devices using the photoelectric conversion effect, for example, light emitting diodes or laser diodes, fuel cells, and the like.
Particularly, a thermoelectric conversion element is used for thermoelectric conversion power generation or thermoelectric conversion cooling applications, and generally includes an N-type thermoelectric semiconductor and a P-type thermoelectric semiconductor electrically connected in series and thermally connected in parallel. The thermoelectric conversion power generation is a method which generates power by converting thermal energy to electrical energy using a thermoelectromotive force generated by creating a temperature difference in a thermoelectric conversion element. Also, the thermoelectric conversion cooling is a method which produces cooling by converting electrical energy to thermal energy using an effect that a temperature difference creates between both ends of a thermoelectric conversion element when a direct current flows through the both ends of the thermoelectric conversion element.
The energy conversion efficiency of the thermoelectric conversion element generally depends on a performance index value or ZT of a thermoelectric conversion material. Here, the ZT may be determined based on the Seebeck coefficient, electrical conductivity, and thermal conductivity, and as a ZT value increases, a thermoelectric conversion material has better performance.
Many thermoelectric materials available for a thermoelectric conversion element have been now proposed and developed, and among them, CuxSe (x≦2) was proposed as a Cu—Se based thermoelectric material and is being developed. This is because CuxSe (x≦2) is known.
Particularly, it has been recently reported that a relatively low thermal conductivity and a high ZT value was achieved in CuxSe (1.98≦x≦2). Typically, Lidong Chen group has reported that Cu2Se exhibited ZT=1.5 at 727° C. (Nature Materials, 11, (2012), 422-425). Also, Gang Chen group of MIT has reported a high ZT value at x=1.96(Cu2Se1.02) and x=1.98(Cu2Se1.01) (x is less than 2) (Nano Energy (2012) 1, 472-478).
However, seeing both of the two results, a comparatively good ZT value was observed at 600° C.˜727° C., but a ZT value was found very low at the temperature lower than or equal to 600° C. Even though a thermoelectric conversion material has a high ZT at a high temperature, if the thermoelectric conversion material has a low ZT value at a low temperature, such a thermoelectric conversion material is not preferred, in particular, unsuitable for a thermoelectric material for power generation. Even if such a thermoelectric material is applied to a heat source of high temperature, a certain region of the material is subjected to a temperature much lower than a desired temperature due to a temperature gradient in the material itself. Therefore, there is a need to develop a thermoelectric material capable of maintaining a high ZT value over a broad temperature range due to having a high ZT value in a temperature range lower than or equal to 600° C., for example, 100° C.˜600° C., as well as in a temperature range higher than 600° C.